In the prior applications noted above, it was pointed out that Asymmetric Digital Subscriber Line (ADSL) is becoming more and more and popular for high-speed modem applications. The ANSI T1.413 ADSL standard uses a technology called Discrete Multi-Tone (DMT) that sends data over 255 separate frequency channels, and each 4 kHz frequency channel can be made to provide a bit rate up to the best present day voice band (33.6 kb/s) modems. This results essentially in overall performance that is equivalent to around two hundred V0.34 modems used in parallel on the same line. Because each channel can be configured to a different bit rate according to the channel characteristics, it can be seen that DMT is inherently "rate-adaptive" and extremely flexible for interfacing with different subscriber equipment and line conditions.
A number of problems arise, however, in attempting to implement a full scale ADSL transceiver cost-effectively, especially in a software modem environment where available signal processing power can vary significantly and unpredictably from device to device. For example, a state of the art desktop computer using the latest microprocessor technology may have a potential signal processing capability many times higher than a simple hand-held computing device. The processor within such devices must also tend to a number of additional operating system and application tasks which limits the available computational time for processing DMT received/transmit symbols. Moreover, DMT technology requires advanced analog front end (AFE) devices that can also push current technology limits and imposes both high cost and power consumption. Both of these facts make a full-scale ADSL implementation undesirable for new and contemplated classes of hand-held personal computing devices. Furthermore, requiring a communications device (such as a modem) to fully support the total throughput of a standard such as ADSL may be unnecessary when prospective users of high-speed data links do not need to use all the available bandwidth provided by such standards.
As disclosed in the above prior applications, an ADSL implementation that permits users to throttle or scale the data throughput in a manner they can control, based on their particular application needs, hardware cost budget, etc., is far more efficient and desirable. As such, one approach discussed at length in the above applications for controlling data bandwidth or throughput is to initiate a link in which the transmitting spectrum of the ADSL signal is confined to a particular set of frequencies (or sub-channels) so that the overall data rate can be restricted to a range suitable for the user setting up the channel. By informing an upstream transceiver that only a selected set of sub-channels should be used, and controlling this sub-set, a user can also thereafter scalably increase the data rate through suitable hardware adjustments, including by adding additional AFE stages that permit a larger section of the ADSL signal to be processed. In this manner, the data rate is scaled by processing a larger and larger portion of a regular ADSL signal within a given ADSL symbol period.
The merits of a scalable data rate ADSL transceiver, therefore, are well known. Nevertheless, the above solution may not be optimal for all possible environments, in the sense that it may not be the simplest, most cost-effective, most flexible, etc. It would also be extremely desirable if it were possible to reduce the effective data rate in other controllable ways which are flexible, easily implementable within the ADSL protocol, and which optimize computational loading on available signal processing circuitry. For example, in an ADSL software modem context, it would be extremely useful if the receive and transmit data rates could be controlled entirely by software updates and modifications, rather than by hardware changes. To date, however, this capability does not exist in prior art ADSL modems.